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   难度：Medium
  </div>
  <div>
   <h1 class="question_title">
    692. Top K Frequent Words
   </h1>
   <p>
    Given a non-empty list of words, return the
    <i>
     k
    </i>
    most frequent elements.
   </p>
   <p>
    Your answer should be sorted by frequency from highest to lowest. If two words have the same frequency, then the word with the lower alphabetical order comes first.
   </p>
   <p>
    <b>
     Example 1:
    </b>
    <br>
   </p>
   <pre>
<b>Input:</b> ["i", "love", "leetcode", "i", "love", "coding"], k = 2
<b>Output:</b> ["i", "love"]
<b>Explanation:</b> "i" and "love" are the two most frequent words.
    Note that "i" comes before "love" due to a lower alphabetical order.
</pre>
   <p>
    <b>
     Example 2:
    </b>
    <br>
   </p>
   <pre>
<b>Input:</b> ["the", "day", "is", "sunny", "the", "the", "the", "sunny", "is", "is"], k = 4
<b>Output:</b> ["the", "is", "sunny", "day"]
<b>Explanation:</b> "the", "is", "sunny" and "day" are the four most frequent words,
    with the number of occurrence being 4, 3, 2 and 1 respectively.
</pre>
   <p>
    <b>
     Note:
    </b>
    <br>
   </p>
   <ol>
    <li>
     You may assume
     <i>
      k
     </i>
     is always valid, 1 &le;
     <i>
      k
     </i>
     &le; number of unique elements.
    </li>
    <li>
     Input words contain only lowercase letters.
    </li>
   </ol>
   <p>
    <b>
     Follow up:
    </b>
    <br>
   </p>
   <ol>
    <li>
     Try to solve it in
     <i>
      O
     </i>
     (
     <i>
      n
     </i>
     log
     <i>
      k
     </i>
     ) time and
     <i>
      O
     </i>
     (
     <i>
      n
     </i>
     ) extra space.
    </li>
   </ol>
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  <div>
   <h1 class="question_title">
    692. 前K个高频单词
   </h1>
   <p>
    给一非空的单词列表，返回前&nbsp;
    <em>
     k&nbsp;
    </em>
    个出现次数最多的单词。
   </p>
   <p>
    返回的答案应该按单词出现频率由高到低排序。如果不同的单词有相同出现频率，按字母顺序排序。
   </p>
   <p>
    <strong>
     示例 1：
    </strong>
   </p>
   <pre>
<strong>输入:</strong> ["i", "love", "leetcode", "i", "love", "coding"], k = 2
<strong>输出:</strong> ["i", "love"]
<strong>解析:</strong> "i" 和 "love" 为出现次数最多的两个单词，均为2次。
    注意，按字母顺序 "i" 在 "love" 之前。
</pre>
   <p>
    &nbsp;
   </p>
   <p>
    <strong>
     示例 2：
    </strong>
   </p>
   <pre>
<strong>输入:</strong> ["the", "day", "is", "sunny", "the", "the", "the", "sunny", "is", "is"], k = 4
<strong>输出:</strong> ["the", "is", "sunny", "day"]
<strong>解析:</strong> "the", "is", "sunny" 和 "day" 是出现次数最多的四个单词，
    出现次数依次为 4, 3, 2 和 1 次。
</pre>
   <p>
    &nbsp;
   </p>
   <p>
    <strong>
     注意：
    </strong>
   </p>
   <ol>
    <li>
     假定
     <em>
      k
     </em>
     总为有效值， 1 &le;
     <em>
      k
     </em>
     &le; 集合元素数。
    </li>
    <li>
     输入的单词均由小写字母组成。
    </li>
   </ol>
   <p>
    &nbsp;
   </p>
   <p>
    <strong>
     扩展练习：
    </strong>
   </p>
   <ol>
    <li>
     尝试以&nbsp;
     <em>
      O
     </em>
     (
     <em>
      n
     </em>
     log
     <em>
      k
     </em>
     ) 时间复杂度和&nbsp;
     <em>
      O
     </em>
     (
     <em>
      n
     </em>
     ) 空间复杂度解决。
    </li>
   </ol>
  </div>
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